Select GN build with an environment variable

tools/generate_buildfiles.py

Index: tools/generate_buildfiles.py
diff --git a/tools/generate_buildfiles.py b/tools/generate_buildfiles.py
new file mode 100644
index 0000000000000000000000000000000000000000..391a15a07ea4358d84c0ed8783ebdaa8b4f7b0b2
--- /dev/null
+++ b/tools/generate_buildfiles.py
@@ -0,0 +1,79 @@
+#!/usr/bin/env python
+# Copyright 2016 The Dart project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import argparse
+import os
+import subprocess
+import sys
+import utils
+
+SCRIPT_DIR = os.path.dirname(sys.argv[0])
+DART_ROOT = os.path.realpath(os.path.join(SCRIPT_DIR, '..'))
+DART_USE_GN = "DART_USE_GN"
+
+
+def use_gn():
+  return DART_USE_GN in os.environ
+
+
+def execute(args):
+  process = subprocess.Popen(args, cwd=DART_ROOT)
+  process.wait()
+  return process.returncode
+
+
+def run_gn():
+  gn_command = [
+    'python',
+    os.path.join(DART_ROOT, 'tools', 'gn.py'),
+    '-m', 'all',
+    '-a', 'all',
+  ]
+  return execute(gn_command)
+
+
+def run_gyp():
+  gyp_command = [
+    'python',
+    os.path.join(DART_ROOT, 'tools', 'gyp_dart.py'),
+  ]
+  return execute(gyp_command)
+
+
+def parse_args(args):
+  args = args[1:]
+  parser = argparse.ArgumentParser(
+      description="A script to generate Dart's build files.")
+
+  parser.add_argument("-v", "--verbose",
+      help='Verbose output.',
+      default=False,
+      action="store_true")
+  parser.add_argument("--gn",
+      help='Use GN',
+      default=use_gn(),
+      action='store_true')
+  parser.add_argument("--gyp",
+      help='Use gyp',
+      default=not use_gn(),
+      action='store_true')
+
+  options = parser.parse_args(args)
+  # If gn is enabled one way or another, then disable gyp
+  if options.gn:
+    options.gyp = False
+  return options
+
+
+def main(argv):
+  options = parse_args(argv)
+  if options.gn:
+    return run_gn()
+  else:
+    return run_gyp()
+
+
+if __name__ == '__main__':
+    sys.exit(main(sys.argv))